1. Field of the Invention
The present invention relates to an etching solution for evaluating crystal defects and, more particularly, to an etching solution used to evaluate a density of Oxidation Induced Stacking faults (to be referred to as OSFs hereinafter) and/or Bulk Microdefects (to be referred to as BMDs hereinafter) present in a silicon wafer.
2. Description of the Related Art
Crystal faults present in a silicon wafer largely affect the characteristics of an element such as a transistor and the like formed on the wafer. In order to obtain good characteristics, crystal defects present in an element region must be eliminated as much as possible. Therefore, in a process technique of manufacturing a semiconductor device, many studies for suppressing generation of crystal defects have been made.
In order to promote such studies, a crystal defects density in a silicon wafer must be precisely, easily, and quickly evaluated. For this purpose, the following method is used. More specifically, a wafer surface is etched using an etching solution having a selectivity with respect to crystal defects such as OSFs and BMDs, and optically detectable pits are formed on a defects portion of the wafer surface. The pits are counted, and the count result is converted to a value per unit volume, thereby evaluating a defects density.
Conventional etching solutions used in the above evaluation method will be described. These etching solutions contain an oxidizing agent for oxidizing silicon and hydrofluoric acid for dissolving silicon dioxide produced upon oxidation of silicon.
Dash etchant
A Dash etchant is an etching solution having a composition ratio of hydrofluoric acid: nitric acid: acetic acid=1:3:12.
The etching character of the Dash etchant does not depend on crystal orientation, and can be advantageously applied to any crystal orientation. However, this solution requires 30 minutes or more for etching. As an etching solution free from the drawback of the Dash etchant, the following etching solution is known.
Sirtl etchant
A Sirtl etchant is an etching solution having a composition ratio of H.sub.2 O: hydrofluoric acid: CrO.sub.3 =1:0.4:0.2. This etching solution can provide a relatively high etching rate of about 1 .mu.m/min. However, this solution has a dependency with respect to crystal orientation. The etching solution can be applied to crystal orientation having indices of face of (111) and (110). However, if the etching solution is applied to crystal orientation having an index of face of (100), pits corresponding to crystal defects cannot be easily formed.
Secco etchant
A Secco etchant is an etching solution consisting of 100 cc of hydrofluoric acid, and 50 cc of a 0.15 mol% aqueous K.sub.2 Cr.sub.2 O.sub.7 solution. This etching solution can provide a relatively high etching rate of about 1.2 .mu.m/min. However, since bubbles are easily attached to a silicon wafer to be etched during etching, ultrasonic cleaning must be performed.
Wright etchant
A Wright etchant is an etching solution consisting of 69 cc of hydrofluoric acid, 30 cc of nitric acid, 30 cc of a 5 mol% aqueous CrO.sub.3 solution, 2 g of copper nitrate, 60 cc of acetic acid, and 60 cc of water. This etching solution can provide a relatively high etching rate of about 1 .mu.m/min. This solution can be applied to any crystal orientation, and the resultant pits have sharp edges and are easy to observe. For this reason, this etching solution is most popular.
As described above, the Sirtl, Secco, and Wright ecthant have satisfactory etching rates but contain hexa-valent chrom (Cr.sup.6+) toxic to a human body. For this reason, problems in respects of operator's health and environmental preservation are posed.